The present invention relates generally to a high efficiency refrigeration system and, more specifically, to a refrigeration system utilizing a single-frequency compressor without ON-OFF operation for variable thermal load for increasing the overall efficiency of a refrigeration system.
FIG. 1 is a block diagram of a conventional refrigeration system, generally denoted at 10. The system includes a compressor 12, a condenser 14, an expansion device 16 and an evaporator 18. The various components are connected together via copper tubing such as indicated at 20 to form a closed loop system through which a refrigerant such as R-12, R-22, R-134a, R-407c, R-410a, ammonia, carbon dioxide or natural gas is cycled.
The main steps in the refrigeration cycle are compression of the refrigerant by compressor 12, heat extraction from the refrigerant to the environment by condenser 14, throttling of the refrigerant in the expansion device 16, and heat absorption by the refrigerant from the space being cooled in evaporator 18. This process, sometimes referred to as a vapor-compression refrigeration cycle, is used in air conditioning systems, which cool and dehumidify air in a living space, or vehicle (e.g., automobile, airplane, train, etc.), in refrigerators and in heat pumps.
FIG. 2 shows the temperature-entropy curve for the vapor compression refrigeration cycle illustrated in FIG. 1. The refrigerant exits evaporator 18 as a superheated vapor at evaporator pressure (Point 1), and is compressed by compressor 12 to a very high pressure. The temperature of the refrigerant also increases during compression, and it leaves the compressor as superheated vapor at condenser pressure (Point 2).
A typical condenser comprises a single conduit formed into a serpentine-like shape with a plurality of rows of conduit lying in a spaced parallel relationship. Metal fins or other structures which provide high heat conductivity are usually attached to the serpentine conduit to maximize the transfer of heat between the refrigerant passing through the condenser and the ambient air. As the superheated refrigerant gives up heat in the upstream portion of the condenser, the superheated vapor becomes a saturated vapor (Point 2a), and after losing further heat as it travels through the remainder of condenser 14, the refrigerant exits as subcooled liquid (Point 3).
As the subcooled liquid refrigerant passes through expansion device 16, its pressure is reduced, and it becomes a liquid-vapor mixture comprised of approximately 20% vapor and 80% liquid. Also, its temperature drops below the temperature of the ambient air as it goes through the expansion device (Point 4 in FIG. 2).
Evaporator 18 physically resembles the serpentine-shaped conduit of the condenser. Air to be cooled is exposed to the surface of the evaporator where heat is transferred to the refrigerant. As the refrigerant absorbs heat in evaporator 18, it becomes a superheated vapor at the suction pressure of the compressor and reenters the compressor thereby completing the cycle (Point 1 in FIG. 2).
One of the challenges in the design and operation of an air-conditioning or refrigeration system is the variation of thermal load over time. The system becomes tremendously inefficient if it is repeatedly turned ON and OFF because there is a significant energy loss associated with the start-up of a compressor. In order to avoid the frequent ON-OFF operations, an inverter compressor is used, which is essentially a variable-speed compressor. Instead of cycling the compressor on and off, the frequency is varied depending on the required thermal load.
FIGS. 3 and 4 show typical performance curves of heat absorption at the evaporator and EER (energy efficiency ratio) versus frequency. FIG. 3 demonstrates the benefit of the inverter-type compressor, which provides 17% more cooling capacity when the frequency increases from the base frequency of 60 Hz to 80 Hz. Furthermore, the cooling capacity decreases by 40% when the frequency decreases from 60 to 30 Hz, an excellent performance from the point of the thermal load variation.
However, the additional cooling capacity of 17% at 80 Hz has its price: there is a severe penalty in the form of a reduced efficiency. As depicted in FIG. 4, there is 18% drop in the EER when the frequency increased from 60 to 80 Hz. Furthermore, the cost of an inverter compressor is often one-third of an air-conditioning or refrigeration system, almost prohibitively expensive for many applications such as room air-conditioners. Thus, a need clearly exists for a way to achieve the benefits of an inverter compressor without the cost and EER penalty.
It is among the objects of this invention:
to provide a refrigeration system using a single-speed compressor which provides the variable cooling capacity of systems using inverter compressors without the disadvantages of such systems;
to provide a refrigeration system in which the compressor can be operated continuously irrespective of heat load;
to provide a variable cooling capacity refrigeration system which does not rely on a costly inverter compressor;
to provide a variable cooling capacity refrigeration system which does not exhibit a significant drop in EER as cooling capacity increases;
to provide a variable cooling capacity refrigeration system suitable for use in room air conditioners;
to provide a method of operating a refrigeration system with increased SEER;
to provide a method of operating a refrigeration system having a single-speed compressor which provides variable cooling capacity without the disadvantages of known variable cooling capacity systems; and
to provide a method of operating a refrigeration system in which a single speed compressor can be run continuously but which provides variable cooling capacity.
SEER, or system energy efficiency ratio, is defined as the ratio of the sum of heat absorption times operation period to the sum of compressor work times operation period. EER (energy efficiency ratio) represents the instantaneous efficiency of a refrigeration system, whereas SEER represents the efficiency of a refrigeration system over an extended period. Use of a continuously operating fixed speed compressor in a variable cooling capacity system has been found to provide improvements in SEER.
According to a first aspect of the invention, the objects of the invention are achieved by providing a variable capacity refrigeration system having condenser means, expansion means, evaporator means and a refrigerant compressor means that operates continuously at a fixed speed when the system is energized, irrespective of the heat load, a refrigerant bypass path that includes secondary expansion means, heat exchanger means, and flow control means. When the heat load is below a predetermined high heat load threshold, the flow control means permits a portion of the refrigerant exiting from the condenser means to flow through the bypass path to an inlet of the compressor means, whereby the heat exchanger means operates as a secondary evaporator means. When the heat load is not below the high heat load threshold, the flow control means prevents refrigerant exiting from the condenser means from flowing through the bypass path to the compressor means.
According to a second aspect of the invention, the objects of the invention are achieved by providing a variable capacity refrigeration system having a condenser, an expansion device, an evaporator and a compressor that operates continuously at a fixed speed when the system is energized, irrespective of the heat load, a refrigerant bypass path that includes a secondary expansion device, a heat exchanger, and a flow control device. When the heat load is above a predetermined high heat load threshold, the flow control device is operable to provide a first cooling capacity. When the heat load is not above the high heat load threshold, the cooling capacity is reduced.
In one embodiment the reduced cooling capacity is provided by diverting a portion of the refrigerant exiting from the condenser to flow through the bypass path to an inlet of the compressor whereby the heat exchanger provides additional subcooling for the condenser. When the heat load is not below the predetermined threshold, the flow control device prevents refrigerant exiting from the condenser to flow through the bypass path to the compressor.
In a variation of the above, the heat exchanger removes heat from the refrigerant flowing through the compressor.
According to a third aspect of the invention, the objects of the invention are achieved by providing a method of operating a refrigeration system to improve the SEER, in which a compressor is operated continuously at a constant speed when the system is energized, irrespective of the heat load, in which there is provided a refrigerant bypass path including a secondary expansion device, a heat exchanger, and a flow control device, and operating the flow control device to provide a first cooling capacity when the heat load is above a predetermined high heat load to threshold, and to provide a reduced second cooling capacity when the heat load is not above the high heat load threshold.
Also according to the first, second, and third aspects of the invention, the refrigerant flow through the bypass path may progressively be increased from the minimum level to a maximum level as the heat load decreases below the high heat load threshold.
When the heat exchanger is operating as a secondary evaporator, warm air may be directed to the heat exchanger and chilled air from the heat exchanger may be directed to the condenser.
Also, when the heat exchanger is operating to reduce the cooling capacity, refrigerant pressure in the heat exchanger may be maintained at a higher level than the pressure in the primary evaporator. In that case, a pressure differential accommodating device reduces the pressure of the refrigerant exiting the heat exchanger. The pressure differential accommodating device may be a vacuum generator such as a vortex generator or a venturi tube, or a flow restrictor such as a capillary tube. When there is no pressure differential between the primary evaporator and the heat exchanger, a pressure differential accommodating device does not have to be used.
According to a variation of the invention as described above, the bypass path may be constructed to operate as a secondary condenser and thereby provide increased cooling capacity for high heat loads. In that case, the flow control device is operable to permit a portion of the refrigerant exiting from the compressor to flow through the bypass path to the primary evaporator through the primary expansion device. Also according to this aspect of the invention, the flow of refrigerant from the compressor outlet through the bypass path may progressively increased from a minimum level to a maximum level as the heat load increases above the high heat load threshold. Further, according to this aspect of the invention, a low heat load threshold may also be selected. When the heat load is below the low heat load threshold, the bypass path operates as a secondary evaporator. Between the two thresholds, the system operates as a conventional system.
In another variation according to the first, second, and third aspects of the invention, heat exchanger is configured to remove heat from the refrigerant exiting the compressor instead of as a secondary evaporator